1. Field of the Invention
The present invention relates to motor control devices that drive and control a motor.
2. Description of Related Art
In order to perform vector control of a motor by supplying three-phase alternating-current power to the motor, it is necessary to detect two phase currents (for example, a U-phase current and a V-phase current) among three phase currents consisting of U-phase, V-phase, and W-phase currents. In general, detection of two phase currents is performed by using two current sensors (such as current transformers). However, the use of two current sensors undesirably increases overall cost of a system incorporating the motor.
It is for this reason that there has conventionally been proposed a technique by which a bus current (direct current) between an inverter and a direct-current power supply is detected with a single current sensor, and, based on the bus current thus detected, two phase currents are detected. This technique is called a single shunt current detection technique.
FIG. 19 is an overall block diagram showing a conventional motor drive system employing the single shunt current detection technique. An inverter (PWM inverter) 902 includes three half-bridge circuits provided one for each of three phases and each having an upper arm and a lower arm. The inverter 902 performs individual switching of each arm in accordance with specified three-phase voltage values fed from a controller 903, and thereby converts a direct-current voltage from a direct-current power supply 904 into a three-phase alternating-current voltage. The three-phase alternating-current voltage thus obtained is supplied to a three-phase permanent-magnet synchronous motor 901 to drive and control the motor 901.
A line connecting each lower arm provided in the inverter 902 with the direct-current power supply 904 is called a bus ML. A current sensor 905 transmits, to the controller 903, a signal indicating a bus current flowing through the bus ML. The controller 903 performs sampling of an output signal of the current sensor 905 with appropriate timing, and thereby detects a phase current of a phase with a maximum voltage level (maximum phase) and a phase current of a phase with a minimum voltage level (minimum phase), namely, two phase currents.
If there is a sufficient level difference among the voltage levels of different phases, two phase currents can be detected in the above-described manner. However, if the voltage levels of the maximum phase and intermediate phase come close to each other, or the voltage levels of the minimum phase and intermediate phase come close to each other, it becomes impossible to detect two phase currents. Incidentally, the single shunt current detection technique will be described later with reference to FIG. 3 to FIGS. 5A to 5D, along with the reason why it becomes impossible to detect two phase currents.
In view of this, there has been proposed a technique of correcting the pulse width of a PWM signal for each arm provided in the inverter based on the three-phase gate signals in the time period during which two phase currents cannot be detected by the single shunt current detection technique.
As an example of such a correction technique, FIG. 20 shows an example of how to correct a specified voltage value (pulse width) in general. In FIG. 20, the horizontal axis represents time, and reference characters 920u, 920v, and 920w represent the voltage levels of the U-phase, the V-phase, and the W-phase, respectively. Since the voltage level of each phase is commensurate with the specified voltage value (pulse width) for each phase, they can be considered equivalent. As shown in FIG. 20, the specified voltage value (pulse width) of each phase is corrected in such a way that a difference between the voltage levels of the “maximum phase and intermediate phase” and a difference between the voltage levels of the “minimum phase and intermediate phase” do not become equal to or smaller than a predetermined value. With this correction, it becomes possible to detect two phase currents with stability. However, as a result of the specified voltage value (pulse width) being corrected, as shown in FIG. 20, distortion of the phase voltage occurs, leading to an undesirable increase in noise and vibration.
It is for this reason that a technique of handling the time period during which two phase currents cannot be detected without correcting the specified voltage value (pulse width) is sought after.
For example, there has been proposed a method for estimating three-phase currents by re-converting d-q axes currents into three-phase currents in the time period during which two phase currents cannot be detected, the d-q axes currents obtained by performing dq conversion on the three-phase currents obtained before that time period.
There has also been proposed a method for allowing the carrier frequency of the PWM inverter to be changed, and reducing the carrier frequency as necessary for making it possible to detect two phase currents. The drawback of these conventional methods, however, is a complicated calculation process that they require.